94 TD 4L80E stuck in 2nd

Hello, new user here. I have a 1994 2500 TD which has been giving me trouble getting stuck in 2nd gear for almost a year. I could shut the truck off and restart and the problem would go away for maybe a week or two. Then it started happening more and more and now it will not reset at all just stuck in 2nd gear. The speedometer also is not working now. I have replaced input and output speed sensors. I checked the codes using the SES light and paper clip and I get a consistent 24. I can reset the codes but when I try to drive then check again the code is back. I noticed that the transmission harness is loose because the plastic clips that hold it together are broke. Dont know how long it was that way. Should I replace the harness before trouble-shooting further? How hard is it to replace? Should I solder or use crimps that come with it? Any suggestions would be greatly appreciated.

The harness is loose or the connector? If you meant the connector, you should definitely replace

First thing you need to do is get the speedometer working right.
Fix all wiring and get it on a transmission/engine capable scanner and read out data and codes.
Post back any code no.